The present invention relates to communication and signal distribution systems and, more particularly, to secure cable television distribution systems which selectively limit access to predetermined channels and maintain the signal quality of channels adjacent to those having limited access.
In cable television distribution systems, it is customary to propagate a selection of basic and premium program signals over a common transmission medium. Access to basic program material is typically provided to all subscribers simply by means of connecting the subscriber's receiver to a converter attached to the cable distribution system. Premium program material, however, is made available for consumption only after special provisions have been made.
Delivery of a strong, undistorted signal of high quality with low equipment and maintenance costs are obvious attributes of cable television distribution systems. Where premium program material is conveyed on a common medium, highly secure and efficient provisions to prevent unauthorized access to premium cable services must also be provided. Such security provisions should exhibit low costs associated with both installation and operation and be controllable from the transmission point to accommodate varying customer demands. In addition, any such security provisions must be operable without a lessening in the quality of the signal delivered and this has been a particular problem with respect to channels adjacent to protected channels and for protected channels delivered to authorized subscribers.
It is known to defeat reception of program material by inserting an interfering carrier within the frequency band of the channel to be blocked during transmission. This interfering carrier may be inserted at the transmission center or the head end and removed at an authorized subscriber's site by filtering techniques or the like, or conversely, selectively inserted at the reception site for unauthorized subscribers.
When an interfering carrier is inserted at the head end, highly precise filtering techniques must be employed for purposes of unblocking a given channel for an authorized subscriber. Such filtering techniques are required since an interfering carrier of this type must be reduced by more than 50 dB to avoid perceptible interference in the reception obtained by an authorized subscriber. Filtering techniques of this type tend to be highly costly, especially in large scale installations servicing a large number of subscribers and it will be apparent that substantial maintenance costs are incurred whenever a subscriber is desirous of changing the nature of the premium channel service purchased.
Conversely, when an interfering carrier is selectively inserted at the reception site, substantial isolation must be provided between adjacent subscribers so that there is no leakage between obstructed and open subscribers. Furthermore, since television signals on a cable may vary 5 to 10 dB, use of such an interfering carrier requires at least 60 dB isolation between subscribers to insure that no leakage between adjacent subscribers occurs. Conventional splitters and the like employed for apportioning cable signals among subscribers do not generally exhibit sufficient isolation for this purpose, and hence, more costly, special purpose devices must be employed.
U.S. Pat. No. 3,859,457 discloses apparatus for selective video suppression which is operative at the subscriber-end on a per subscriber basis. In this reference, a notch filter tuned to the spectrum of a video signal to be selectively inhibited is employed. The filter exhibits relatively little attenuation, however, the signal passed is modulated at a rate exceeding the response capacity of the automatic gain control circuit present in a television receiver. The resulting amplitude modulated signal is essentially unrecoverable in that receiver synchronization, color subcarrier recovery and overall video reception are precluded. In practice, however, it is costly and difficult to provide a filter having a notch deep enough to capture sufficient signal while being sufficiently narrow to preserve adjacent channels. Multiple traps might be cascaded for several channels, however, this results in high insertion loss and would, of course, increase equipment costs which are already high.
It is therefore an object of the present invention to provide improved secure cable television distribution systems which are highly cost effective.
It is an additional object of the present invention to provide secure cable television distribution systems wherein adjacent channel interference is minimized.
It is another object of the present invention to provide secure television distribution systems wherein access to premium channel information at subscriber sites is controlled from the transmission point and is readily variable.
It is a further object of the present invention to provide secure cable television distribution systems wherein premium channel information being conveyed is processed and directly employed to prevent reception at selected subscriber sites.
Various other objects and advantages of the present invention will become clear from the following detailed description of several preferred embodiments thereof and the novel features will be particularly pointed out in conjunction with the claims appended hereto.